Human-cytomegalovirus (HCMV) is a significant human pathogen. Primary infection and reactivation of HCMV causes severe diseases in neonates and immuno-compromised individuals. Previously, we have cloned HCMV strains AD 169 and Toledo as infectious bacterial artificial chromosomes (BAC), developed an efficient BAC-based HCMV genetic system and constructed a comprehensive collection of HCMV AD 169 mutants using a systematic mutagenesis scheme. Initial characterization of the HCMV mutant library has generated a body of information that allows us to start to delineate functions of many previously uncharacterized viral genes. The long term goal of this research project is to understand the mechanisms by which HCMV inhibits programmed cell death, i.e. apoptosis, in virus infection. Apoptosis is one of the critical host defenses in response to virus infection to eliminate virus replication and spread;HCMV has evolved strategies to antagonize apoptosis for its replication, persistence and dissemination in the host. Over-expressions of several HCMV proteins (i.e. IE1, IE2, pUL36 and pUL37x1) have been shown to inhibit apoptosis. We have taken advantage of the AD169 mutant library to directly identify the HCMV protein pUL38 as a novel cell death suppressor that is required for efficient virus infection in human fibroblasts. We hypothesize that pUL38 plays an important role in preventing cell death and facilitating virus replication in various cell types instrumental to HCMV pathogenesis. In Specific Aim 1, we will define the biological role of pUL38 and its murine cytomegalovirus homolog (i.e., pM38) in virus infection in various cell culture models. In Specific Aim 2, we will use the mutational approach to define the sequence basis for the ability of pUL38 to block cell death in virus infection. In Specific Aim 3, we will investigate the mechanisms by which pUL38 interferes with cellular signaling pathways to block apoptosis. The proposed studies will lead to a better understanding of the complex apoptosis paradigm during CMV infection and set the stage for using the mouse model of MCMV infection as a valuable alternative to study pUL38 in CMV pathogenesis in vivo. Ultimately, these virus-encoded apoptosis suppressors have the potential to serve as candidate targets for developing novel therapeutic interventions to control this globally important pathogen.